1. Field of the Invention
The present invention relates to airline optical fibers, and in particular relates to an airline optical fiber with reduced multipath interference (MPI), and methods of forming same.
2. Technical Background
Optical fibers are widely used in a variety of applications, including the telecommunications industry in which optical fibers are employed in a number of telephony and data transmission applications. Due, at least in part, to the extremely wide bandwidth and the low noise operation provided by optical fibers, the use of optical fibers and the variety of applications in which optical fibers are used are continuing to increase. For example, optical fibers no longer serve as merely a medium for long distance signal transmission, but are being increasingly routed directly to the home or, in some instances, directly to a desk or other work location.
Airline (or “air line”) optical fibers (“AL fibers”) have one or more regions with either aperiodically or periodically arranged small holes or voids, which make the fiber extremely bend insensitive. AL fibers with aperiodically arranged holes are referred to as “random airline” or RAL fibers. Examples of airline optical fibers are described in, for example, U.S. Pat. No. 6,243,522, pending U.S. patent application Ser. No. 11/583,098 filed Oct. 18, 2006, and provisional U.S. patent application Ser. Nos. 60/817,863 filed Jun. 30, 2006; 60/817,721 filed Jun. 30, 2006; 60/841,458 filed Aug. 31, 2006; 60/841,490 filed Aug. 31, 2006; and 60/879,164, filed Jan. 8, 2007 (hereinafter, “the Corning airline fiber patents and patent applications”), all of which are assigned to Corning Incorporated and all of which are incorporated by reference herein.
One type of AL fiber has an annular ring of airlines (the “airline ring”) surrounding the core and that extends longitudinally down the length of the fiber. The airline ring has a reduced apparent or average index of refraction because air has an index of refraction of ˜1 compared to the surrounding fused silica matrix refractive index of ˜1.45. The airline ring is positioned to create a refractive index profile that enables superior bend performance (optically) and significantly smaller minimum bend radius specifications.
One drawback of AL fibers is that the airline ring can trap higher order modes. These higher order modes can cause multi-path interference (MPI) in a fiber system. For an AL fiber longer than about 22 m, MPI is typically not an issue because there is a cable cutoff wavelength of 1260 nm that ensures the higher-order modes are attenuated. However, for short sections of AL fibers (“jump fibers”) of less than a few meters in length, MPI can generate power fluctuations and noise that reduce system performance. It is therefore desirable to reduce the MPI that can occur in AL fibers without compromising their superior fiber bending performance.